Distribution is one of those issues where an implementation has to be opinionated, and weigh out the pros and cons of each implementation. How do you handle partition events? How do you handle conflicts between two nodes, especially when resolving one conflict can generate other conflicts. How do you handle latency? How do you handle failover?

Extensions like this are how Postgres has traditionally handled interesting opinionated technologies. The mailing lists will mull over the various implementations, scratch their chin a lot, and finally come to a conclusion after much discussion. It's why the database is so strong and stable, and why some seemingly obvious features take a while to get into the core database.

Because Postgres is conservative with new features and they prefer not having a feature (and the option to include it later) to being stuck supporting a not perfect attempt.

Why would it be, postgres is not a distributed database?

pg_paxos is brand new and while the current release is reasonably stable and tested under various failure scenarios on EC2, it's also lacking in some areas, especially performance (think 10-100 writes/s). We will continue developing it to make it faster, easier to deploy, and address many interesting applications. Eventually we might submit it to postgres, but it's easier to develop it as an extension for now.